Mount Sinai researchers engineer 'smart' mRNA that targets cancer cells while sparing healthy tissue

Scientists at the Icahn School of Medicine at Mount Sinai have engineered a new form of messenger RNA that does something previous therapies could not: it recognizes the difference between a cancer cell and a healthy one, and turns itself on or off accordingly. The system, called the cell-selective modRNA translation system, or cSMRTS, was tested in mice and published in November in the journal Molecular Therapy. If the approach proves safe and effective in humans, it could reshape how mRNA therapies work—moving from a blunt instrument that floods the body with medicine to a precision tool that knows exactly where to act.

The problem cSMRTS solves is real and stubborn. When mRNA vaccines were developed for COVID-19, it did not matter which cells in the body produced the spike protein; the goal was simply to train the immune system. But cancer treatment is different. A drug that kills tumor cells is only useful if it actually reaches tumor cells and leaves everything else alone. Current delivery methods—tiny fat bubbles called lipid nanoparticles—can be engineered to find their way to certain tissues, but they are imprecise. Some of the medicine leaks into healthy organs. Some tumors are missed. The researchers wondered whether they could flip the problem on its head: instead of making the delivery vehicle smarter, what if they made the mRNA itself smart?

The cSMRTS system works like a two-part lock. Cancer cells produce distinctive molecules called microRNAs that healthy cells do not. The engineered mRNA carries two separate instructions. The first codes for an enzyme called Cas6, and it includes a target site that cancer-related microRNAs can recognize and attack. The second carries the actual therapeutic gene, wrapped in a small RNA loop that Cas6 can cut. In a cancer cell, the microRNAs find and disable the Cas6 instructions, so Cas6 is never made—and without Cas6, the therapeutic gene stays intact and turns on. In a healthy cell, where those cancer-specific microRNAs are absent, Cas6 gets made normally, finds the therapeutic gene, cuts it, and shuts the treatment down. The cancer cell's own molecular signature becomes the key that unlocks the medicine.

When the Mount Sinai team tested this in mice with breast and colon tumors, the selectivity was striking. Gene activity in the tumors was more than one hundred times higher than in healthy tissue. In major organs like the liver and spleen, activity dropped by more than three hundred and eighty times. Using a tumor-suppressor gene called Pten, the researchers saw tumor growth slow by forty-five percent. When they combined cSMRTS with an mRNA-based immunotherapy, tumors shrank by up to ninety-three percent.

What makes this advance potentially transformative is its flexibility. The system is not locked into one disease or one type of therapy. The same basic architecture—using cell-specific microRNAs as a control switch—could theoretically be adapted to target inflammatory cells in autoimmune disease, metabolic cells in diabetes, or cardiac cells in heart disease. One of the senior researchers noted that in his fifteen years studying mRNA in the heart, he has relied on injecting medicine directly into the organ to avoid harming surrounding tissue. A system that could target heart cells selectively without invasive procedures would be a fundamental shift.

The researchers have filed patent applications and are now moving toward commercialization and further preclinical work. The path from mouse studies to human trials is long and uncertain. But the core insight—that mRNA can be engineered to make its own decisions about where to act—opens a door that has been closed. If cSMRTS works in people the way it works in mice, it could mean cancer patients eventually receive treatments that are not just more powerful but more precise, with fewer side effects and less collateral damage to the body. It could also mean that mRNA, which has so far been most useful as a vaccine platform, finally becomes the flexible therapeutic tool its inventors always imagined.